PURPOSE: To improve assembly workability by connecting an imput gear and a multiplate type joint device via an inner member, loosely engaging rotatably an output gear with the outer periphery of the inner member, and separably connecting the joint device and the input and output gears in a shaft direction via claw clutches arranged inside and outside.

CONSTITUTION: An inner member 3a and an output gear 2d are combinatively inserted in and spline-engaged with one end of an input gear 2c fixed to a main shaft 3. Then, when an outer member is supported by the main shaft 3, while the friction plate 4c of a friction joining means 4a is engaged with the inner member 3a, and successively first and second supporting members 2f and 2h are engagedly inserted in the friction plate 4c; a claw clutch 6j facilitates the joining between the output gear 2d and the pressure receiving plate 4h of the friction joining means 4a, also between the braking cylinder 6h of a parking brake 6 and the pressure receiving plate 4h of a steering baking means 4b.